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S.M. Couch (Butler Univ.), H.N. Cohn, P.M. Lugger (Indiana Univ.), A.M. Cool (San Francisco State Univ.), J. Anderson (Rice Univ.)
We have searched for additional optical identifications of Chandra X-ray sources in the nearby (d = 2.5 kpc), collapsed-core globular cluster NGC 6397, using Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) imaging in H\alpha, R, and B from the GO-10257 dataset. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified two additional faint CV candidates, three candidate active binaries (likely BY Dra type), and one candidate active galactic nucleus. We detect possible optical counterparts for two additional sources, although the photometry does not permit object classification in these cases. Of the 25 Chandra sources detected within the half-mass radius of NGC 6397 by Grindlay et al. (2001, ApJ, 563, L53), only four remain unclassified. The 11 likely CVs in NGC 6397 split into two groups: six brighter CVs for which the optical emission is dominated by contributions from the secondary and accretion disk and five fainter CVs for which the white dwarf dominates the optical emission. The faintest CVs have properties consistent with the WZ Sge class. The spatial distribution of the brighter CVs is much more centrally concentrated than that of the fainter CVs. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.
This work is supported by NASA grant HST-GO-10257A and NSF REU grant AST-0452975.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.